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How long would it take to accelerate a spacecraft to 99 per cent light speed without major injury to the astronauts inside?
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Ron Dippold
San Diego, California, US
It wouldn’t be long if you had infinite energy! Let’s lock down our units – 99 per cent of the speed of light, or 0.99c, is 1068 billion kilometres per hour. Normal Earth gravity acceleration, 1 g, is equivalent to 9.8 metres per second squared – every second, you go about 10 metres per second faster. You have to take time compression into effect: time passes slower for people on board because they are going faster (explained by general relativity).
The time it takes to get to 99 per cent light speed is about 81 million seconds per g, or 2.6 years on-board time. To someone watching on Earth, it would be 6.8 years. So if you could accelerate infinitely without worrying about fuel or running into anything, it would only take you two and a half years to get to 99 per cent light speed at a perfectly comfortable 1 g.
How high can you crank up the acceleration? There have been a few studies on the long-term effects of high gravity on people, and they suggest people can comfortably tolerate 1.25 g to 1.5 g. Above that, the fluids in your body start draining to places they aren’t supposed to before too long. At 1.5 g, it would take 1.7 years on-board time to reach 99 per cent light speed. A normal person without special training or equipment can withstand about 5 g without passing out if in a chair. That would be half a year of misery though! Trained people with special compression suits can do 9 g, which would only take 0.3 years. So perhaps if you strapped your hypernauts into a compression vat and kept them fed and hydrated with hoses and amused in virtual reality, you could get there in three and a half months at 9 g.
If you could accelerate infinitely, it would only take you two and a half years to get to 99 per cent light speed at a comfortable 1 g
The real problem is that there is no technology that would let us accelerate at a constant 1 g for two and a half years. Besides the obvious collision issues (even a speck of dust could be lethal), you have to carry enough fuel to accelerate the fuel you are carrying, which means you need more fuel, and so on. This is why the Saturn V rocket was so gigantic, and that would be nothing compared to this. A Falcon 9 rocket masses about 550,000 kilograms at launch – a back-of-the-envelope calculation suggests that you’d need about 4 × 10115000 kilograms of standard rocket fuel to get it to 0.99c, which is 4 followed by 115,000 zeros. Our entire galaxy is only estimated to be 3 × 1042 kilograms! This is why science fiction with travel between stars needs warp drives or external propulsion sources.
Mike Follows
Sutton Coldfield, West Midlands, UK
The safest acceleration would mimic the gravitational field strength we experience on Earth’s surface – essentially, free-fall acceleration. Rather than being pulled downwards by Earth’s gravity, we would feel the same force (our weight) if we stood on a surface accelerating upwards at the same rate as free-fall.
A spacecraft with an upward acceleration of 1 g, taking relativity into account, would take about 2.6 years to reach 99 per cent of the speed of light. Once the spacecraft reaches a constant speed, it could be converted into an O’Neill cylinder, which generates artificial gravity on its inner surface by spinning about its axis of symmetry. The required rearrangement of the internal layout of the spacecraft brings to mind The Twits by Roald Dahl when Mr and Mrs Twit were led to believe that their house had been turned upside down.
Pat French
Longdon-upon-Tern, Shropshire, UK
The questioner wishes to avoid injury to the occupants of the craft. This would be difficult, not to say impossible. As the vehicle approached light speed, those occupants would find themselves in a hole, and quite a deep one.
As they neared the speed of light, their mass would approach infinity relative to the bodies by which they passed. Planets and stars would be drawn into the abyss in space-time created by such a mass. This near-infinite mass would collide with dust and particles, with near-infinite, catastrophic impact. It would not be a comfortable journey.
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